The present invention relates to clock generators, and more specifically, it relates to a clock generator for deriving clock information from a digital bit stream.
According to a digital modulation technique, such as eight-to-fourteen modulation (EFM), original eight bit codes of random clock spacings are converted to fourteen bit codes of the format having a minimum spacing of three clock pulses and a maximum spacing of eleven clock pulses to provide enough clock information to demodulate the EFM bit stream.
Conventional clock generators employed for demodulating such EFM bit streams include circuitry that detects a series of data bits having the minimum and maximum clock spacings and counts clock pulses generated by a voltage-controlled oscillator which are present during the minimum and maximum spacings of the detected data bits. Two count values are derived as a measure of the frequency of the clock pulse to control the frequency of the oscillator. One disadvantage of the prior art is that since the data bits of minimum and maximum clock spacings occur at random the phase-locked loop is likely to remain out-of-phase with the input bit stream for a substantial period if successive frames contain no data bits having mininum and maximum clock spacings.